The invention relates to a multilayer capacitor composed of alternate layers of a dielectric oxidic ceramic material and of an electrode material, formed into a compact unit by the application of heat and pressure.
Multilayer capacitors of the above-described type (so-called monolithic ceramic capacitors) have so far generally been manufactured on an industrial scale as follows.
A slurry of finely ground ceramic dielectric powder mixed with a binder is deposited in thin layers which are dried to form foils and the foils are then provided with electrodes by means of silk-screening a metal paste on them. The electroded foils are stacked and compressed and severed into separate capacitor bodies. Said capacitor bodies are sintered at temperatures between 1200.degree. and 1400.degree. C., depending on the composition of the ceramic, dielectric material. During sintering the ceramic, dielectric foils shrink and assume a dense polycrystalline structure, the density of the sintered material being larger than that of the original foils. Simultaneously the powder particles of the metal paste sinter to form electrode layers of metal which form a coherent assembly with the dielectric layers.
In practice, however, it is difficult to make a stack of perfect thin layers. Short-circuits at low voltages may occur, these are ascribed to too high a porosity and to the occurrence of cracks and delaminations, i.e. the non-adherence of layers in the final product, respectively.
The porosity could be restricted by causing the sintering to take place while simultaneously applying pressure, so-called pressure-sintering. In a commonly-owned copending patent application Ser. No. 656,219, filed Oct. 1, 1984 it is disclosed that if said pressure is not applied on all sides (isostatically) but in a direction transverse to the plane of the layer (uniaxially), the occurrence of delaminations could also be prevented. This teaching is herein incorporated by reference. Experiments which have led to the present invention, however, have demonstrated than when a uniaxial pressure-sintering process is used to manufacture multilayer capacitors, a new problem may occur: the final products sometimes do not satisfy the requirements imposed because the metal of the electrode layers tends to mix to a greater or lesser extent with the oxidic ceramic material of the dielectric layers. This is ascribed to the condition that as a result of the high pressures which are exerted during the uniaxial pressure-sintering process, the flow limit of the metal of the electrode layers is surpassed. The metal which in these conditions is comparatively soft is then easily pressed into the ceramic material which at the beginning of the sintering has not yet reached its maximum density. This pressure problem becomes more serious as the thickness of the ceramic layers decreases.